The invention proceeds from a motor vehicle air spring system having a level control arrangement. Air spring systems with level control arrangements are known in various configurations.
U.S. Pat. No. 5,142,897 discloses an arrangement for level control for a vehicle having air springs. The elevation signals of the elevation sensors are filtered at a time constant in order to improve the control performance of the arrangement. An elevation change is either caused by a load change or by a leak in an air spring. In control operations, usually there is no differentiation as to whether a control request takes place because of a changed load state or because of a leakage.
It is an object of the invention to provide a method for detecting especially medium size leaks in a motor vehicle air spring system, that is, with a control, for example, the desired level can still be reached.
The essence of the arrangement according to the invention is a function logic for detecting leakages in the level-controlled air spring system. This function logic is part of the level control arrangement. The operation of this logic is based on a separate treatment of the level control in different driving states.
The operation of the function logic will now be described.
(a) Ignition is Switched Off.
In this state, the level control apparatus switches off after an after-run time of, for example, five minutes. Thereafter, the control apparatus (SG) awakens time controlled, for example, after two hours, and then, for example, after five hours again in order to evaluate the control deviation. If it is detected that the vehicle is too low at least at one vehicle corner (lower threshold value elevation level reached or there is a drop therebelow), the vehicle is controlled at all vehicle corners to the desired level within the tolerance limits or threshold values. Thereafter, the control apparatus switches into the sleep mode.
(b) Ignition of the Vehicle is Switched On.
A continuous level monitoring takes place by the control apparatus (SG). If the tolerance limits are exceeded (for example, there is a drop below the threshold value hsu) the vehicle is again controlled to the desired level.
With respect to (a), the detection logic according to the invention for detecting leakages in the individual air springs during control apparatus after-run.
When the ignition is switched off (KL15-off) and after the elapse of a fixed time span (for example, five minutes), the control apparatus goes into the sleep mode (current save mode). Before the control apparatus goes into the sleep mode, the air pressure p1 for each air spring (VL, VR, HL, HR) and the distance h1 (VL, VR, HL, HR) is measured in the air springs from wheel to wheel at time point t1 in accordance with the method of the invention and is stored together with the time t1. With a renewed awakening of the control apparatus (movement out of the current save mode), only with the detection of an up control requirement at time point t2 (that is, there is a drop below the lower threshold value hsu), the air pressures p2 (VL, VR, HL, HR) are measured in the individual air springs.
For specific circuit arrangements, it is especially advantageous to only then determine the air pressures p2 when there is a drop below the lower threshold value hsu of the elevation level at least one vehicle corner. In these circuit arrangements, an air exchange can take place with the corresponding lines and, if required, with the connected air drive for each pressure measurement in an air spring insofar as a pressure drop exists between the pressure in the air spring bellows and the components connected for the pressure measurement. The volume of an air dryer amounts normally to approximately 200 to 300 cm3. If the air dryer, for example, has a pressure close to atmospheric pressure, then a drop of the elevation level at the vehicle corner of this air spring bellows is possible when there is an air exchange and a pressure compensation with the connected air spring bellows. In an unfavorable case, a drop of the vehicle corner could take place and lead to a detection of leakage during the determination of a leakage in the sequence of first carrying out a pressure measurement and then determining the elevation level because of the above-described air exchange and pressure compensation with the connected lines and components. In the determination of the leakage in the reverse sequence, this is reliably avoided (first determining the elevation level and then, when necessary, making the pressure measurement) whereby pressurized air and energy are saved.
For evaluating the condition, that air spring is used for the evaluation at whose axle a control requirement was detected and that air spring is selected which lies the lowest, that is, which has the lowest value of h2. The conditions for the air spring selected in this manner are as follows:
Condition 1: (p1xe2x88x92p2) greater than K1 wherein K1xe2x89xa70 (Wheel load has not increased, for example, K1=0 bar) and
Condition 2: {h(t1)xe2x88x92h(t2)}*K3/(t2xe2x88x92t1) greater than K2 wherein K2 greater than 0 and K3 greater than 0 (Elevation level at the corresponding air spring has not decreased, for example, K2=1/s and K3=10/mm) and,
Condition 3: (t2xe2x88x92t1) less than TLimit (Time condition in which a leak can be detected).
If the above conditions are satisfied, a leakage at this air spring is detected and a wheel-specific counter ZT is incremented. If, in contrast, no control requirement is detected at t2 (that is, there is no drop below threshold value hsu of the corresponding air spring), the corresponding wheel-specific counters ZT (VL, VR, HL, HR) are decremented, preferably, minimally to the value 0.
At TLimit, it can be inputted how large the leak should be so that this leak can still be detected. The greater TLimit is, the lower the leakage values which still can be detected and the lower TLimit is, the greater the leakage has to be in order to still be detected via the conditions.
If a wheel-specific counter ZT exceeds a threshold value K4 (for example, the value 4), a leakage fault is detected. A leakage fault is displayed optically and/or acoustically in the interior of the vehicle. The particular leakage fault can be displayed wheel-specifically and/or as a fault for the entire air spring system. A leakage fault is preferably stored in the control apparatus in a permanent memory in order to make it available for later analysis and repair purposes.
For multiple awakening operations, the method is correspondingly repeated in that the values at the instantaneous time point are again assigned to the time point t2 and to the past time point, the time point t1. That is, as a new time point t1, the last time point t2 is selected at which the control apparatus was transferred from the active state into the sleep mode. The new time point t2 is selected time-controlled after a pregiven time span of, for example, three hours when the control apparatus is again transferred out of the sleep mode into the active state.
Variations:
The time point t1 does not have to be necessarily determined with the first transfer of the control apparatus after the switchoff of the ignition (KL15=off) into the sleep mode; instead, the time point t1 can be determined also, for example, with the first awakening of the control apparatus (leaving the sleep mode) and the detection of a control need, that is, there is a drop below the threshold value hsu of at least one air spring. The time point t2 shifts then to the next awakening of the control apparatus. The reason is that a separately initiated pressure measurement causes disturbing noises. At the start of each control operation, however, a pressure measurement is anyway carried out. With this variation, no additional pressure measurements (valve switching) are therefore necessary. Furthermore, effects because of the cooling of the air springs can be precluded in that t1 is so selected that the air spring is cooled down.
With respect to condition (b), the detection logic of the invention for detecting leaks in the air spring bellows with the ignition switched on are now discussed.
A condition precedent for the activation of the subsequent logic is that an up-control operation was triggered for the switched-on ignition (KL15=An) which was not triggered because of a change of the desired level (manual level adjustment). If such a control operation took place at time point t1, the air pressures p1 in the air spring bellows and the time point t1 are stored at the beginning of the control operation or with the detection of a drop below threshold value hsu. The time point t2 identifies the time point when the next control requirement is detected, that is, there is a drop below threshold value hsu and an up-control operation is triggered. At this time point t2, before the start of the up control, the pressures p2 are measured. When the conditions 1 and 3 are satisfied, the corresponding wheel-individual counter is incremented. To evaluate the leakage, that air spring is used in accordance with the method of the invention at whose axle the up control requirement was detected and that air spring is selected at which the vehicle body is at the lowest with respect to elevation, that is, where the lowest value of h2 is present.
The wheel-individual counters are time controlled decremented with a value greater than xcex94t=t2xe2x88x92t1, for example, every hour (therefore, xcex94t=1 h) or xcex94t=2*(t2xe2x88x92t1), or when a leakage is not detected at the corresponding air spring, if in this time span, or during the leakage detection, the valves of the corresponding air springs were not actuated. Preferably, the wheel-specific counters are decremented down to the value 0.
If a wheel-individual counter exceeds a threshold value K4, a leakage fault is detected which is allocated to the corresponding air spring.
According to the invention, the monitoring takes place only for pressure changes in order to exclude level changes because of changes of load. In order to eliminate influences caused by temperature, the time span between t2 and t1 is to be so selected that elevation changes caused by temperature influences still do not lead to exceeding the control hysteresis, that is, this does not lead to a crossing of the upper and lower threshold values hso and hsu.
Compared to air springs which are provided with a conventionally functioning level control arrangement, the method and arrangement of the invention afford the advantage that air spring leakages can be detected for the first time. Only an elevation sensor in the individual air spring bellows and a pressure sensor are conditions precedent.
A further advantage is that not only the basic detection of a leak in the arrangement is possible; rather a localization of the leak can also be undertaken, for example, air spring forward left (VL), et cetera.
It is further advantageous that a wheel-specific counter is incremented with the detection of a leak at an air spring. In this way it is possible to monitor the time-dependent course of the wheel-specific leakage.
A still further advantage is that a leakage fault is detected at an air spring only after a threshold value is exceeded. In this way, a defective detection of leakage because of elevation signal fluctuations caused, for example, by load changes and vibration excitations during travel or temperature fluctuations is substantially avoided.
A further advantage of the invention is that the pressure measurement is only undertaken after the evaluation of the elevation signals. There must be a drop below the lower elevation threshold value at least at one vehicle corner. In this way, pressurized air and energy are saved.
Finally, an indication as to a leak and/or a leakage fault can be stored in a permanent fault memory of the control apparatus for the purpose of providing information at a service center.
Acoustic or visual display of the leak in the interior of the vehicle for the driver is advantageous. The driver is informed directly as to the state of the level control arrangement and can take corresponding safety relevant steps, for example, by driving more slowly.